Optical fiber winding for measuring the current circulating through a conductor, and optical fiber-based current measuring equipment

ABSTRACT

An optical fiber winding for measuring current circulating through a conductor. The optical fiber winding includes a first helically wound optical fiber cable and a second helically wound optical fiber cable. The first helically wound optical fiber cable is twisted about its longitudinal axis in a first twist direction, and the second helically wound optical fiber cable is twisted about its longitudinal axis in a second twist direction, the first twist direction being opposite the second twist direction. Each of the first and second helically wound optical fiber cables making contact with one another at multiple locations along their length. Due to the first and second helically wound optical fiber cables making contact with one another and being twisted in opposite directions, counteracting forces exist where the first and second helically wound optical fiber cables contact one another to resist an untwisting.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/340,462, filed Jun. 7, 2021, which relates to and claims the benefitand priority to European Application No. EP20382515.3, filed Jun. 15,2020.

TECHNICAL FIELD

The present invention relates to an optical fiber winding and opticalfiber-based current measuring equipment for measuring the currentcirculating through a conductor.

BACKGROUND

Measuring equipment based on optical fiber for measuring the currentcirculating through a conductor is known. Said equipment operatesaccording to the Faraday effect, i.e., the magnetic field generated bythe current circulating through the conductor causes a rotation in thepolarization of the light circulating through the optical fiber windingarranged around the conductor. The operation of said measuring equipmentconsists of emitting light through optical fiber to the optical fiberwinding in which the characteristics of the light are modified dependingon the current circulating through the conductor, and analyzing therotation caused in the polarization of light so as to determine themagnitude of the current circulating through the conductor.

Theoretically speaking, the polarization of light of a perfectlycircular single-mode optical fiber in which said light is propagated ina longitudinal propagation direction remains unchanged if it receives noexternal actions of any type. That is, theoretically speaking, thepolarization of light at the inlet of the optical fiber would beidentical to the polarization of light at the outlet of the opticalfiber. Therefore, in the event of the action of a magnetic fieldgenerated by the current circulating through a conductor, the rotationin the polarization of light would only be due to the currentcirculating through the conductor.

However, in the real world, the polarization of light within an opticalfiber presents a fairly random behavior which hinders measuring current.There are various factors that can be attributed to the optical fiberitself, causing the polarization of light in the longitudinalpropagation direction to be changed. For example, the core of theoptical fiber is not perfectly circular; when the optical fiber isarranged around the conductor mechanical stresses which are nothomogeneous along its longitudinal extension are created; optical fiberis an element with an elastic behavior which can be twisted and bent.All of this causes changes in the polarization of light.

To correct these problems relative to the random behavior of thepolarization of light within the optical fiber, special optical fibersensors or specific measurement methods minimizing the impact generatedby said randomness are known to be used.

EP3104183A1 or EP3598149A1, both belonging to the same applicant as thepresent invention, disclose optical fiber-based current measuringequipment for measuring the current circulating through a conductor,comprising an interrogator with light-emitting means and light-receivingmeans, a sensing portion close to the conductor, and an optical fibertransmission means arranged between the interrogator and the sensingportion.

SUMMARY

Disclosed is an optical fiber winding and optical fiber-based currentmeasuring equipment for measuring the current circulating through aconductor.

A first aspect of the invention relates to an optical fiber winding formeasuring the current circulating through a conductor, comprising:

-   -   a central support core extending in a longitudinal direction,    -   a first optical fiber cable arranged around the central support        core, and    -   a second optical fiber cable arranged around the central support        core.

The optical fiber cables extend in a helical manner around the centralsupport core, the first optical fiber cable is twisted about itslongitudinal axis in a first twist direction, and the second opticalfiber cable is twisted about its longitudinal axis in a second twistdirection, the first twist direction being opposite the second twistdirection.

A second aspect of the invention relates to optical fiber-based currentmeasuring equipment for measuring the current circulating through aconductor, comprising:

-   -   an interrogator with light-emitting means and light-receiving        means,    -   a sensing portion close to the conductor, and    -   an optical fiber transmission means arranged between the        interrogator and the sensing portion, wherein the sensing        portion comprises an optical fiber winding comprising:        -   a central support core extending in a longitudinal            direction,        -   a first optical fiber cable arranged around the central            support core, and        -   a second optical fiber cable arranged around the central            support core.

The optical fiber cables extend in a helical manner around the centralsupport core, the first optical fiber cable is twisted about itslongitudinal axis in a first twist direction, and the second opticalfiber cable is twisted about its longitudinal axis in a second twistdirection, the first twist direction being opposite the second twistdirection.

The twisting of the optical fiber allows single-mode optical fibercables to be used for measuring the current circulating through theconductor. The twisting of the optical fiber is one of the factorschanging the polarity of the light circulating through the opticalfiber, such that by applying twisting on the longitudinal axis of thefiber, the remaining factors changing the polarity of light can beconsidered negligible with respect to the twisting factor. Therefore, byintroducing controlled twisting in the optical fiber allows for thechanges in the polarization of light basically being due to the magneticfield caused by the current circulating through the conductor, where theother factors can be considered irrelevant for the measurement of thecurrent.

However, optical fiber is an elastic element which tends to recover itsoriginal shape when twisting is no longer applied. According to theinvention, the two optical fiber cables twisted in opposite directionsand arranged around the central support core allow obtaining a stablemechanical assembly which can be readily wound around the conductor thecurrent of which is to be measured. The natural tendency of opticalfiber cables is to return to their natural position, but since thecables are twisted in opposite directions, they will make efforts torecover their shape which will be counteracted.

A winding which is free of mechanical stresses and can be used as anoptical fiber assembly for measuring the current of a conductor isthereby obtained. By means of a winding or wire drawing machine, theoptical fiber winding can be arranged around the conductor, with theoptical fibers of the winding remaining twisted around the centralsupport core, without loops or bends arising which hinder the placementof the winding on the conductor.

These and other advantages and features will become apparent in view ofthe figures and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first embodiment of an optical fiber winding formeasuring the current circulating through a conductor.

FIG. 2 shows a second embodiment of an optical fiber winding formeasuring the current circulating through a conductor.

FIG. 3 shows a third embodiment of an optical fiber winding formeasuring the current circulating through a conductor.

FIG. 4 shows a fourth embodiment of an optical fiber winding formeasuring the current circulating through a conductor.

FIG. 5 shows optical fiber-based current measuring equipment formeasuring the current circulating through a conductor.

DETAILED DESCRIPTION

The invention relates to an optical fiber winding 100 arranged around aconductor 5 for measuring the current I circulating through theconductor 5. The optical fiber winding 100 operates according to theFaraday effect. The magnetic field generated by the current Icirculating through the conductor 5 causes a rotation in thepolarization of the light circulating through the optical fiber winding100 arranged around the conductor 5.

As shown in the embodiments of FIGS. 1 to 4 , the optical fiber winding100 comprises a central support core 110 extending in a longitudinaldirection Z, a first optical fiber cable 120 arranged around the centralsupport core 110, and a second optical fiber cable 130 arranged aroundthe central support core 110.

The optical fiber cables 120 and 130 extend in a helical manner aroundthe central support core 110 in the longitudinal direction Z. The firstoptical fiber cable 120 is twisted about its longitudinal axis Z1 in afirst twist direction T1, and the second optical fiber cable 130 istwisted about its longitudinal axis Z2 in a second twist direction T2,the first twist direction T1 being opposite the second twist directionT2.

Twisting is understood as the optical fiber cables 120 and 130 beingrotated about their longitudinal axes a certain number of turns.

The material with which the optical fiber cables are made has an elasticbehavior, such that when a force is applied on the cable, the shape ofthe cable is modified, but when is removed said force, the elasticbehavior of the cable tends to return it to its natural position. Theconfiguration of the optical fiber winding 100 of the invention solvesthat problem. The central support core 110 keeps the optical fibercables 120 and 130 extended in the longitudinal direction Z, impedingthe natural tendency of the fiber to go back to its natural position.Furthermore, since the cables 120 and 130 are twisted in oppositedirections, they make efforts to recover their shape that arecounteracted. All this allows a mechanical stability of the opticalfiber winding 100 without the need to apply external forces to maintainsaid stability.

For example, to maintain the twisting of the optical fiber cables 120and 130, a force can be applied at each end of the cable and in theopposite direction, i.e., the optical fiber cables 120 and 130 can belongitudinally stressed from their ends to keep them twisted. However,applying that longitudinal tension causes the optical fiber cable toacquire an elliptical instead of circular shape, i.e., linearbirefringence increases, which causes the measurement capacity to bereduced. The previously described arrangement of the winding 100 allowsmaintaining the twisting of the cables 120 and 130 without having toapply longitudinal tension in the cables.

FIG. 5 shows an example of optical fiber-based current measuringequipment for measuring the current circulating through a conductor 5.The equipment comprises an interrogator 1 with light-emitting means 2and light-receiving means 3, a sensing portion 4 close to the conductor5, and optical fiber transmission means 6 arranged between theinterrogator 1 and the sensing portion 4. The sensing portion 4comprises an optical fiber winding 100 such as the one previouslydescribed, which is arranged around the conductor 5 for measuring thecurrent I circulating through the conductor 5.

The operation of the measuring equipment consists of emitting lightthrough the optical fiber transmission means 6 to the optical fiberwinding 100 in which the features of the light are modified depending onthe current I circulating through the conductor 5, and analyzing therotation caused in the polarization of the light so as to determine themagnitude of the current I circulating through the conductor 5.

Pulses of light are sent from the light-emitting means 2 of theinterrogator 1 to the optical fiber winding 100 of the sensing portion 4through the optical fiber transmission means 6. The pulses are reflectedin reflection means 7, such as a Faraday mirror, or a normal mirror, andreturn through the optical fiber winding 100, being directed towards thelight-receiving means 3 of the interrogator 1 through the optical fibertransmission means 6.

The interrogator 1 is located away from the sensing portion 4 formeasuring the current I remotely, and the optical fiber transmissionmeans 6 allow connecting the interrogator 1 with the sensing portion 4at large distances.

FIG. 5 illustrates the optical fiber winding 100 arranged in a helicalmanner around the conductor 5. Alternatively, the optical fiber winding100 can be arranged on an optical fiber reel. The winding 100 istherefore not arranged in a helical manner on the conductor 5, butrather the winding 100 is wound up a certain number of turns forming aloop inside the reel, and the conductor 5 is passed through the centerof said reel. In that configuration, the reel is arranged transverselywith respect to the longitudinal direction in which the conductorextends.

FIGS. 1 and 2 show first and second embodiments of the optical fiberwinding 100.

The first optical fiber cable 120 extends in a helical manner around thecentral support core 110 according to a first rotation direction R1, andthe second optical fiber cable 130 extends in a helical manner aroundthe central support core 110 according to a second rotation directionR2, the first rotation direction R1 being opposite the second rotationdirection R2, such that the optical fiber cables 120,130 are interlacedwith one another around the central support core 110. Therefore, everytime the cables 120 and 130 are interlaced, a contact area is createdbetween both which favors the cables maintaining the twisting and theirstability on the core central 110.

In the first embodiment of FIG. 1 , the first twist direction T1 of thefirst cable 120 is counterclockwise, the first rotation direction R1 ofthe first cable 120 is counterclockwise, the second twist direction T2of the second cable 130 is clockwise, and the second rotation directionR2 of the second cable 130 is clockwise.

In the second embodiment of FIG. 2 , the first twist direction T1 of thefirst cable 120 is clockwise, the first rotation direction R1 of thefirst cable 120 is counterclockwise, the second twist direction T2 ofthe second cable 130 is counterclockwise, and the second rotationdirection R2 of the second cable 130 is clockwise.

FIGS. 3 and 4 show third and fourth embodiments of the optical fiberwinding 100. The first optical fiber cable 120 extends in a helicalmanner around the central support core 110 according to a first rotationdirection R1, and the second optical fiber cable 130 extends in ahelical manner around the central support core 110 according to a secondrotation direction R2, the first rotation direction R1 being the same asthe second rotation direction R2, and the optical fiber cables 120 and130 being in contact with one another along their entire longitudinalextension around the central support core 110. Unlike the embodiments ofFIGS. 1 and 2 , in this case the cables 120 and 130 are not interlacedwith one another, such that retaining points between them are notcreated; however, the cables 120 and 130 are at all times in contactwith the central support core 110 and with one another.

In the third embodiment of FIG. 3 , the first twist direction T1 of thefirst cable 120 is counterclockwise, the second twist direction T2 ofthe second cable 130 is clockwise, and the first rotation direction R1of the first cable 120 and the second rotation direction R2 of thesecond cable 130 are counterclockwise.

In the fourth embodiment of FIG. 4 , the first twist direction T1 of thefirst cable 120 is clockwise, the second twist direction T2 of thesecond cable 130 is counterclockwise, and the first rotation directionR1 of the first cable 120 and the second rotation direction R2 of thesecond cable 130 are clockwise.

Preferably, the first optical fiber cable 120 is twisted about itslongitudinal axis Z1 a first number of turns, and the second opticalfiber cable 130 is twisted about its longitudinal axis Z2 a secondnumber of turns, the first and the second number of turns being thesame. The twisting of both cables is therefore the same, and thebehavior of the polarization of light in both cables is thereforesimilar. This feature allows applying methods for measuring current inwhich it is necessary for both cables to emit optical signals in orderto carry out the measurement.

Preferably, the optical fiber cables 120 and 130 are twisted

${12 \times 2\pi\frac{rad}{m}},$i.e., the cables are twisted 12 certain turns for each meter of cable.

The central support core 110 can be a mechanical element, such as ametal or plastic cable. Alternatively, the central support core 110 ismade up of one or more optical fiber cables. This arrangement allowsemitting an additional number of optical signals which can be used forapplying different methods for measuring the current I circulatingthrough the conductor 5.

The optical fiber cables 120 and 130 are standard single-mode opticalfiber cables of the type normally used in the telecommunication sector.The twisting of the optical fiber cables 120 and 130 allows being ableto use single-mode optical fiber cables to perform the measurement ofthe current of the conductor 5. Therefore, by using standard opticalfiber cables a low-cost optical fiber winding 100 is obtained to performcurrent measurements, which is particularly relevant when the current inlong cables for carrying electrical energy is to be measured. Forexample, the optical fiber winding 100 may be 100 meters long.

A single-mode optical fiber cable generally comprises a core, a claddingsurrounding the core, a buffer surrounding the cladding, and a jacketsurrounding the buffer. The core is the physical component that carrieslight and is generally made of glass fiber, the cladding protects thecore and allows the propagation of the light. The buffer and the jacketare elements generally made of rigid plastic which provide protectionand stiffness to the optical fiber cable.

As explained above, the polarization of light within an optical fiberpresents a fairly random behavior which hinders measuring current. If anoptical fiber is analyzed, from the point of view of propagationthereof, for purposes of polarization, and without contemplating thepresence of magnetic fields due to the action exerted by an electricalconductor, the following behavior matrix is obtained:

$\begin{bmatrix}E_{xo} \\E_{yo}\end{bmatrix} = {\begin{bmatrix}{{\cos\left( {\sigma l} \right)} + {j\frac{\delta_{l}}{2\sigma}{\sin\left( {\sigma l} \right)}}} & {\frac{\left( {\psi + \frac{\delta_{c}}{2}} \right)}{\sigma}{\sin\left( {\sigma l} \right)}} \\{{- \frac{\left( {\psi + \frac{\delta_{c}}{2}} \right)}{\sigma}}{\sin\left( {\sigma l} \right)}} & {{\cos\left( {\sigma l} \right)} - {j\frac{\delta_{l}}{2\sigma}\sin\left( {\sigma l} \right)}}\end{bmatrix}\begin{bmatrix}E_{xi} \\E_{yi}\end{bmatrix}}$wherein:

$\begin{bmatrix}E_{xo} \\E_{yo}\end{bmatrix}$is the electric field at the outlet of the optical fiber.

$\begin{bmatrix}E_{xi} \\E_{yi}\end{bmatrix}$is the electric item et me inlet of the optical fiber.δ_(i) is the linear birefringence of the optical fiber in rad/m.δ_(c) is the circular birefringence of the optical fiber in rad/m.ψ is the twisting of the optical fiber in rad/m.l is the length of the optical fiber in m.

$\sigma = \sqrt{\left( \frac{\delta_{l}}{2} \right)^{2} + \left( {\psi + \frac{\delta_{c}}{2}} \right)^{2}}$

In the presence of a magnetic field due to the action exerted by anelectrical conductor, the behavior matrix would be:

$\begin{bmatrix}E_{xo} \\E_{yo}\end{bmatrix} = {\begin{bmatrix}{{\cos\left( {\sigma l} \right)} + {j\frac{\delta_{l}}{2\sigma}{\sin\left( {\sigma l} \right)}}} & {\frac{\left( {\psi + \phi + \frac{\delta_{c}}{2}} \right)}{\sigma}{\sin\left( {\sigma l} \right)}} \\{{- \frac{\left( {\psi + \phi + \frac{\delta_{c}}{2}} \right)}{\sigma}}{\sin\left( {\sigma l} \right)}} & {{\cos\left( {\sigma l} \right)} - {j\frac{\delta_{l}}{2\sigma}\sin\left( {\sigma l} \right)}}\end{bmatrix}\begin{bmatrix}E_{xi} \\E_{yi}\end{bmatrix}}$wherein ϕ=VBlϕ is the angle of rotation of the polarization of light in radians.V is the Verdet constant of the optical fiber.B is the density of the magnetic field.l is the length of the optical fiber in m.

In the behavior matrix of an optical fiber with respect to polarization,it can be observed that if sufficient twisting is applied to the fiber,that term twisting ψ becomes dominant, such that the polarization matrixis simplified:

$\begin{bmatrix}E_{xo} \\E_{yo}\end{bmatrix} = {\begin{bmatrix}{\cos\left( {\sigma l} \right)} & {\sin\left( {\sigma l} \right)} \\{- {\sin\left( {\sigma l} \right)}} & {\cos\left( {\sigma l} \right)}\end{bmatrix}\begin{bmatrix}E_{xi} \\E_{yi}\end{bmatrix}}$

The change in circular birefringence providing twisting to the fiber,which is about −0.075 of the amount of twisting performed, isdisregarded.

In the presence of a magnetic field, the following holds:σ≈(ψ+ϕ)

Given that the twisting may be unstable once certain levels of magnitudeare reached, the light is required to pass through the turns of thewinding 100, to be reflected in the mirror 7 and to go back through saidturns of the winding 100, such that the effect of mechanical twisting iscancelled out, and given that the effect of the magnetic field is notsymmetrical, the magnetic field measurement is doubled.

In this manner and by placing the mirror 7 at the end final of theoptical fiber winding 100 (for example, a Faraday mirror, or a normalmirror, or another reflective element), the polarization matrix wouldbe: This is the polarization matrix using a Faraday mirror:

$\begin{bmatrix}E_{xo} \\E_{yo}\end{bmatrix} = {\begin{bmatrix}{- {\sin\left( {2\phi} \right)}} & {\cos\left( {2\phi} \right)} \\{\cos\left( {2\phi} \right)} & {\sin\left( {2\phi} \right)}\end{bmatrix}\begin{bmatrix}E_{xi} \\E_{yi}\end{bmatrix}}$

It can therefore be observed that with the fiber twisted, the changepolarization, is due only to the presence of the magnetic field.

According to the foregoing, on one hand, the invention proposes twistingthe optical fiber cables 120 and 130 to preferably be able to usestandard single-mode optical fiber cables for measuring current. On theother hand, the manner in which the optical fiber winding 100 isperformed allows maintaining the twisting of the optical fiber cables120 and 130 without the need to use external forces or elements.

Embodiments of optical fiber windings and optical fiber-based currentmeasuring equipment are also disclosed in the following clauses.

Clause 1. Optical fiber winding for measuring the current circulatingthrough a conductor, comprising:

-   -   a central support core (110) extending in a longitudinal        direction (Z),    -   a first optical fiber cable (120) arranged around the central        support core (110), and    -   a second optical fiber cable (130) arranged around the central        support core (110), wherein the optical fiber cables (120, 130)        extend in a helical manner around the central support core        (110), the first optical fiber cable (120) is twisted about its        longitudinal axis (Z1) in a first twist direction (T1), and the        second optical fiber cable (130) is twisted about its        longitudinal axis (Z2) in a second twist direction (T2), the        first twist direction (T1) being opposite the second twist        direction (T2).

Clause 2. Winding according to clause 1, wherein the first optical fibercable (120) extends in a helical manner around the central support core(110) according to a first rotation direction (R1), and the secondoptical fiber cable (130) extends in a helical manner around the centralsupport core (110) according to a second rotation direction (R2), thefirst rotation direction (R1) being opposite the second rotationdirection (R2), such that the optical fiber cables (120,130) areinterlaced with one another around the central support core (110).

Clause 3. Winding according to clause 2, wherein the first twistdirection (T1) of the first cable (120) is counterclockwise, the firstrotation direction (R1) of the first cable (120) is counterclockwise,the second twist direction (T2) of the second cable (130) is clockwise,and the second rotation direction (R2) of the second cable (130) isclockwise.

Clause 4. Winding according to clause 2, wherein the first twistdirection (T1) of the first cable (120) is clockwise, the first rotationdirection (R1) of the first cable (120) is counterclockwise, the secondtwist direction (T2) of the second cable (130) is counterclockwise, andthe second rotation direction (R2) of the second cable (130) isclockwise.

Clause 5. Winding according to clause 1, wherein the first optical fibercable (120) extends in a helical manner around the central support core(110) according to a first rotation direction (R1), and the secondoptical fiber cable (130) extends in a helical manner around the centralsupport core (110) according to a second rotation direction (R2), thefirst rotation direction (R1) being the same as the second rotationdirection (R2), and the optical fiber cables (120,130) being in contactwith one another along their entire longitudinal extension around thecentral support core (110).

Clause 6. Winding according to clause 5, wherein the first twistdirection (T1) of the first cable (120) is counterclockwise, the secondtwist direction (T2) of the second cable (120) is clockwise, and thefirst rotation direction (R1) of the first cable (130) and the secondrotation direction (R2) of the second cable (130) are counterclockwise.

Clause 7. Winding according to clause 5, wherein the first twistdirection (T1) of the first cable (120) is clockwise, the second twistdirection (T2) of the second cable (130) is counterclockwise, and thefirst rotation direction (R1) of the first cable (120) and the secondrotation direction (R2) of the second cable (130) are clockwise.

Clause 8. Winding according to any of the preceding clauses, wherein thefirst optical fiber cable (120) is twisted about its longitudinal axis(Z1) a first number of turns, and the second optical fiber cable (130)is twisted about its longitudinal axis (Z2) a second number of turns,the first and second number of turns being the same.

Clause 9. Winding according to the preceding clause, wherein the opticalfiber cables (120,130) are twisted

$12 \times 2\pi{\frac{rad}{m}.}$

Clause 10. Winding according to any of the preceding clauses, whereinthe central support core (110) is made up of one or more optical fibercables.

Clause 11. Optical fiber-based current measuring equipment for measuringthe current circulating through a conductor, comprising:

-   -   an interrogator (1) with light-emitting means (2) and        light-receiving means (3),    -   a sensing portion (4) close to the conductor (5), and    -   an optical fiber transmission means (6) arranged between the        interrogator (1) and the sensing portion (4),

the sensing portion (4) comprises an optical fiber winding (100)comprising:

-   -   a central support core (110) extending in a longitudinal        direction (Z),    -   a first optical fiber cable (120) arranged around the central        support core (110), and    -   a second optical fiber cable (130) arranged around the central        support core (110),    -   wherein the optical fiber cables (120, 130) extend in a helical        manner around the central support core (110), the first optical        fiber cable (120) is twisted about its longitudinal axis (Z1) in        a first twist direction (T1), and the second optical fiber cable        (130) is twisted about its longitudinal axis (Z2) in a second        twist direction (T2), the first twist direction (T1) being        opposite the second twist direction (T2).

Clause 12. Equipment according to clause 11, wherein the first opticalfiber cable (120) extends around the central support core (110)according to a first rotation direction (R1), and the second opticalfiber cable (130) extends in a helical manner around the central supportcore (110) according to a second rotation direction (R2), the firstrotation direction (R1) being opposite the second rotation direction(R2), such that the optical fiber cables (120,130) are interlaced withone another around the central support core (110).

Clause 13. Equipment according to clause 11, wherein the first opticalfiber cable (120) extends around the central support core (110)according to a first rotation direction (R1), and the second opticalfiber cable (130) extends in a helical manner around the central supportcore (110) according to a second rotation direction (R2), the firstrotation direction (R1) being the same as the second rotation direction(R2), and the optical fiber cables (120,130) being in contact with oneanother along their entire longitudinal extension around the of thecentral support core (110).

Clause 14. Equipment according to any of clauses 11 to 13, wherein thefirst optical fiber cable (120) is twisted about its longitudinal axis(Z1) a first number of turns, and the second optical fiber cable (130)is twisted about its longitudinal axis (Z2) a second number of turns,the first and second number of turns being the same.

Clause 15. Equipment according to the preceding clause, wherein theoptical fiber cables (120,130) are twisted

$12 \times 2\pi{\frac{rad}{m}.}$

What is claimed is:
 1. An optical fiber winding for measuring thecurrent circulating through a conductor, the optical fiber windingcomprising: a first helically wound standard single-mode optical fibercable having a length and a first central axis, the first helicallywound standard single-mode fiber cable being twisted about the firstcentral axis in a first twist direction; and a second helically woundstandard single-mode optical fiber cable having a length and a secondcentral axis, the second helically wound standard single-mode opticalfiber cable being twisted about the second central axis in a secondtwist direction, the second twist direction being opposite the firsttwist direction, each of the first and second helically wound standardsingle-mode optical fiber cables making contact with one another atmultiple locations along their length, due to the first and secondhelically wound standard single-mode optical fiber cables making contactwith one another and being twisted in opposite directions, counteractingforces exist where the first and second helically wound standardsingle-mode optical fiber cables contact one another to resist anuntwisting of the first and second helically wound standard single-modeoptical fiber cables.
 2. The optic fiber winding according to claim 1,further comprising a central support core, each of the first and secondhelically wound standard single-mode optical fiber cables being arrangedaround the central support core.
 3. The optical fiber winding accordingto claim 2, wherein the first helically wound standard single-modeoptical fiber cable extends in a helical manner around the centralsupport core according to a first rotation direction, and the secondhelically wound standard single-mode optical fiber cable extends in ahelical manner around the central support core according to a secondrotation direction, the first rotation direction being opposite thesecond rotation direction, such that the first and second helicallywound standard single-mode optical fiber cables are interlaced with oneanother around the central support core.
 4. The optical fiber windingaccording to claim 3, wherein the first twist direction of the firsthelically wound standard single-mode optical fiber cable iscounterclockwise, the first rotation direction of the first helicallywound standard single-mode optical fiber cable is counterclockwise, thesecond twist direction of the second helically wound standardsingle-mode optical fiber cable is clockwise, and the second rotationdirection of the second helically wound standard single-mode opticalfiber cable is clockwise.
 5. The optical fiber winding according toclaim 3, wherein the first twist direction of the first helically woundstandard single-mode optical fiber cable is clockwise, the firstrotation direction of the first helically wound standard single-modeoptical fiber cable is counterclockwise, the second twist direction ofthe second helically wound standard single-mode optical fiber cable iscounterclockwise, and the second rotation direction of the secondhelically wound standard single-mode optical fiber cable is clockwise.6. The optical fiber winding according to claim 2, wherein the firsthelically wound standard single-mode optical fiber cable extends in ahelical manner around the central support core according to a firstrotation direction, and the second helically wound standard single-modeoptical fiber cable extends in a helical manner around the centralsupport core according to a second rotation direction, the firstrotation direction being the same as the second rotation direction, andthe first and second helically wound standard single-mode optical fibercables being in contact with one another along their entire longitudinalextension around the central support core.
 7. The optical fiber windingaccording to claim 6, wherein the first twist direction of the firsthelically wound standard single-mode optical fiber cable iscounterclockwise, the second twist direction of the second helicallywound standard single-mode optical fiber cable is clockwise, and thefirst rotation direction of the first helically wound standardsingle-mode optical fiber cable and the second rotation direction of thesecond helically wound standard single-mode optical fiber cable arecounterclockwise.
 8. The optical fiber winding according to claim 6,wherein the first twist direction of the first helically wound standardsingle-mode optical fiber cable is clockwise, the second twist directionof the second helically wound standard single-mode optical fiber cableis counterclockwise, and the first rotation direction of the firsthelically wound standard single-mode optical fiber cable and the secondrotation direction of the second helically wound standard single-modeoptical fiber cable are clockwise.
 9. The optical fiber windingaccording to claim 1, wherein the first helically wound standardsingle-mode optical fiber cable is twisted about the first central axisin the first twist direction a first number of turns, and the secondhelically wound standard single-mode optical fiber cable is twistedabout the second central axis in the second twist direction a secondnumber of turns, the first and second number of turns being the same.10. The optical fiber winding according to claim 9, wherein each of thefirst and second helically wound standard single-mode optical fibercables is twisted $12 \times 2\pi{\frac{rad}{m}.}$
 11. The optical fiberwinding according to claim 2, wherein the central support core is madeup of one or more optical fiber cables.
 12. An optical fiber-basedcurrent measuring assembly for measuring the current circulating througha conductor, the assembly comprising: an interrogator including a lightemitter and a light receiver; a sensing portion close to the conductor;and an optical fiber transmission means arranged between theinterrogator and the sensing portion, the sensing portion including anoptical fiber winding comprising: a first helically wound standardsingle-mode optical fiber cable having a length and a first centralaxis, the first helically wounds standard single-mode fiber cable beingtwisted about the first central axis in a first twist direction; and asecond helically wound standard single-mode optical fiber cable having alength and a second central axis, the second helically wound standardsingle-mode optical fiber cable being twisted about the second centralaxis in a second twist direction, the second twist direction beingopposite the first twist direction, each of the first and secondhelically wound standard single-mode optical fiber cables making contactwith one another at multiple locations along their length, due to thefirst and second helically wound standard single-mode optical fibercables making contact with one another and being twisted in oppositedirections, counteracting forces exist where the first and secondhelically wound standard single-mode optical fiber cables contact oneanother to resist an untwisting of the first and second helically woundstandard single-mode optical fiber cables.
 13. The optic fiber windingaccording to claim 12, further comprising a central support, each of thefirst and second helically wound standard single-mode optical fibercables being arranged around the central support core.
 14. The opticalfiber-based current measuring assembly according to claim 13, whereinthe first helically wound standard single-mode optical fiber cableextends in a helical manner around the central support core according toa first rotation direction, and the second helically wound standardsingle-mode optical fiber cable extends in a helical manner around thecentral support core according to a second rotation direction, the firstrotation direction being opposite the second rotation direction, suchthat the first and second helically wound standard single-mode opticalfiber cables are interlaced with one another around the central supportcore.
 15. The optical fiber-based current measuring assembly accordingto claim 14, wherein the first twist direction of the first helicallywound standard single-mode optical fiber cable is counterclockwise, thefirst rotation direction of the first helically wound standardsingle-mode optical fiber cable is counterclockwise, the second twistdirection of the second helically wound standard single-mode opticalfiber cable is clockwise, and the second rotation direction of thesecond helically wound standard single-mode optical fiber cable isclockwise.
 16. The optical fiber-based current measuring assemblyaccording to claim 14, wherein the first twist direction of the firsthelically wound standard single-mode optical fiber cable is clockwise,the first rotation direction of the first helically wound standardsingle-mode optical fiber cable is counterclockwise, the second twistdirection of the second helically wound standard single-mode opticalfiber cable is counterclockwise, and the second rotation direction ofthe second helically wound standard single-mode optical fiber cable isclockwise.
 17. The optical fiber-based current measuring assemblyaccording to claim 13, wherein the first helically wound standardsingle-mode optical fiber cable extends in a helical manner around thecentral support core according to a first rotation direction, and thesecond helically wound standard single-mode optical fiber cable extendsin a helical manner around the central support core according to asecond rotation direction, the first rotation direction being the sameas the second rotation direction, and the first and second helicallywound standard single-mode optical fiber cables being in contact withone another along their entire longitudinal extension around the centralsupport core.
 18. The optical fiber-based current measuring assemblyaccording to claim 17, wherein the first twist direction of the firsthelically wound standard single-mode optical fiber cable iscounterclockwise, the second twist direction of the second helicallywound standard single-mode optical fiber cable is clockwise, and thefirst rotation direction of the first helically wound standardsingle-mode optical fiber cable and the second rotation direction of thesecond helically wound standard single-mode optical fiber cable arecounterclockwise.
 19. The optical fiber-based current measuring assemblyaccording to claim 17, wherein the first twist direction of the firsthelically wound standard single-mode optical fiber cable is clockwise,the second twist direction of the second helically wound standardsingle-mode optical fiber cable is counterclockwise, and the firstrotation direction of the first helically wound standard single-modeoptical fiber cable and the second rotation direction of the secondhelically wound standard single-mode optical fiber cable are clockwise.20. The optical fiber-based current measuring assembly according toclaim 12, wherein the first helically wound standard single-mode opticalfiber cable is twisted about the first central axis in the first twistdirection a first number of turns, and the second helically woundstandard single-mode optical fiber cable is twisted about the secondcentral axis in the second twist direction a second number of turns, thefirst and second number of turns being the same.
 21. The opticalfiber-based current measuring assembly according to claim 20, whereineach of the first and second helically wounds standard single-modeoptical fiber cables is twisted $12 \times 2\pi{\frac{rad}{m}.}$